Aluminum, as a metallization material, has been one of the key factors in the success of solid-state semiconductor circuits. It readily reduces the native oxide on silicon surfaces at low temperatures (&lt;500.degree. C.) and thus forms excellent contact with silicon and, for the same reasons, bonds very well with silicon dioxide (SiO.sub.2) and SiO.sub.2 -based glasses. However, incorporating aluminum in emergency ultra-large scale integration (ULSI) computer chip devices has encountered several problems. One is material reliability due to aluminum's low melting point. Aluminum reacts strongly with silicon and easily migrates through silicon. Another problem is process reliability. Present physical vapor deposition (PVD) processes, of which sputtering is the most popular, cannot meet the increasingly stringent requirements of new multilevel metallization schemes. Sputtering produces non-conformal coverage, which leads to thinning at via and trench edges and walls, and to keyholes in the via. In addition, the deposits, grown at or near room temperature, almost invariably are contaminated with trapped-in sputter gas and possess small grain size. Both features are detrimental to the reliability of aluminum interconnections. Higher temperature deposition solves some of these problems. However, thermally-fragile low dielectric constant (.epsilon.&lt;2) polymers, which are considered for applications as interlayer and passivating dielectric to enhance the performance of integrated circuits, are destroyed during high temperature processes.
In spite of these problems, aluminum's use is expected to continue in ULSI and beyond, as documented by the Semiconductor Industry Association (SIA) industry wide technology roadmap. See The National Technology Roadmap for Semiconductors (SIA, San Jose, Calif., 1994). This expectation is contingent upon the development of new aluminum alloys and deposition techniques which eliminate the inherent problems encountered in PVD processes.
Chemical vapor deposition (CVD) potentially offers a solution to all these problems. CVD deposits a thin solid film synthesized from the gaseous phase by a chemical reaction which could be activated thermally or electrically and/or catalyzed by the substrate to be coated. It is this reactive process which distinguishes CVD from physical deposition processes, such as sputtering or evaporation. CVD is used to deposit layers of silicon, silicon dioxide and silicon nitride. CVD is not used to deposit metals on semiconductor substrates. One of the key advantages of CVD is its potential ability to involve the substrate surface in the deposition reaction which leads, under the proper conditions, to a conformal, planarized blanket, or selective metal growth. This conformed feature is an essential requirement to produce three-dimensional multilevel structures which contain interconnections in the vertical direction through vias and holes in the dielectric layers. Another advantage of CVD is that it can deposit layers on substrates of complex shape and form the layers at growth rates which are much higher than the minimum acceptable in electronic device industry. In addition, it can grow metal thin films at reduced temperatures, as low as 150.degree. C., with no need for post-deposition annealing. This is necessary to minimize the effects of interdiffusion and to allow the growth of abrupt multilayered structures. It is relatively simple and controllable, and leads to good adherence, high uniformity over a large area, and reduced susceptibility to interfacial mixing and cross-contamination effects.
In recent years, considerable efforts have been devoted to the development of CVD processes for depositing aluminum layers on substrates, in particular semiconductor substrates. Earlier attempts at aluminum CVD used tri-alkyl-type sources, such as trimethyl and triethylaluminum, and produced deposits with extensive surface roughness, high resistivity, and large amounts of carbon, all of which being detrimental to microelectronic applications. See, e.g., C. F. Powell, J. H. Oxley, and J. M. Blocher, Jr., Vapor Deposition (Wiley, New York, N.Y. 1966) p. 277; and H. J. Cooke, R. A. Heinecke, R. C. Stern, and J. W. C. Maas, Solid State Technol. 25 (1982) 62. Also, the pyrophoric nature of the alkyl source precursors required extensive precautionary measures. These earlier attempts used relatively high temperatures, increased reactor pressure, and did not use hydrogen.
To avoid these problems, attempts were made to grow aluminum through hydrogen reduction of aluminum halides, such as AlCl.sub.3 and AlBr.sub.3, or through disproportionation of aluminum subchlorides. See, e.g., W. Klemm, E. Voss, and K. Geigersberger, Z. Anorg. Allg. Chemie 256 (1948) 15; and A. S. Russel, K. E. Martin, and C. N. Cochran, Am. Chem. Soc. 73 (1951) 1466. Precursor transport to the reaction zone required however prohibitively high temperatures (&gt;700.degree. C.) and made the process impractical.
More recently, several reports were published on the formation of a sensitizing layer on SiO.sub.2 prior to aluminum CVD and on the use of new organoaluminum source precursors, such as triisobutylaluminum (TIBA) and trimethylamine-alane (TMAAl). See, e.g., R. A. Levy, P. K. Gallagher, R. Contolini, and F. Schrey, J. Electrochem. Soc. 132 (1985) 457; B. E. Bent, R. G. Nuzzo, and L. H. Dubois, J. Am. Chem. Soc. 111 (1989) 1634, and H. O. Pierson, Thin Solid Films, 45 (1977) 257; M. E. Gross, K. P. Cheung, C. G. Fleming, J. Kovalchick, and L. A. Heimbrok, J. Vac. Sci. Technolo. A9 (1991) 1; M. E. Gross, L. H. Dubois, R. G. Nuzzo, and K. P. Cheung, Mat. Res. Symp. Proc., Vol 204 (MRS, Pittsburgh, Pa., 1991) p. 383; W. L. Gladfelter, D. C. Boyd, and K. F. Jensen, Chemistry of Mater. 57 (1989) 339; D. B. Beach, S. E. Blum, and F. K. LeGoues, J. Vac. Sci. Technol. A7 (1989) 3117. In addition, NTT in Japan announced the development of a multilayer wiring technique based on a selective Al CVD process. However, the process requires high vacuum capabilities of rf plasma pre-cleaning for in-situ impurity removal from the inner surface of the via holes.
In spite of all attempts, only a few of which have been cited here, low-temperature (&lt;475.degree. C.) CVD of device-quality aluminum is not yet feasible. Some particular problems encountered include prohibitive surface roughness, impurity contamination (especially oxygen and carbon which bond well to aluminum), high deposition temperature, and the lack of sensitized layers that allows precursor decomposition on initial, insulating, surfaces. In addition, as discussed below, prior CVD methods fail to provide device-quality aluminum and aluminum-copper alloys with conformal step coverage for substrates having aggressive holes and trenches (i.e., with a diameter of 0.25 .mu.m .mu.m or smaller) and high aspect ratios (i.e., the ratio of hole depth to hole width equal to or greater than about 4:1).
So, there is a long felt, critical need for a process and apparatus to provide specular and pure aluminum and doped aluminum (aluminum with a few percent of other elements, such as copper) films suitable for ULSI fabrication. A typical, specular aluminum film has a grain size below a few thousand angstroms. Such films must be of ultra high quality, in terms of purity, with impurity concentrations well below 1 atomic percent, must exhibit excellent electromigration properties, must be highly specular, with extremely smooth surface morphology, and must be conformal to the complex topography of ULSI circuity to provide complete filling of aggressive via and trench structures. The desired process and apparatus should readily prepare single films containing either aluminum or copper doped aluminum, as well as bilayer films of aluminum and copper, and that such technology be amenable to process temperatures below about 475.degree. C. to prevent thermally induced devices damage during processing.
Copper doping is required to enhance aluminum's resistance to electromigration. This could be achieved through sequential deposition of aluminum then copper, followed by annealing or rapid thermal processing (RTP) to alloy the two films and produce a homogeneous copper-doped aluminum phase. However, work was recently published on the CVD formation of aluminum films doped with 0.7-1.4 wt % copper through the simultaneous decomposition in the same CVD reactor of dimethylaluminum hydride (DMAH) and cyclopentadienyl copper triethylphosphine which were employed, respectively, as the aluminum and copper sources. See T. Katagiri, E. Kondoh, N. Takeyasu, T. Nakano, H. Yamamoto, and T. Ohta, Jpn. J. Appl. Phys. 32 (1993)LI078 and J. Electrochem. Soc. 141 (1994) 3494. Unfortunately, the copper source used in the work was highly reactive and unstable during transport and handling, which makes it undesirable for real industrial applications. The references fail to disclose plasma assisted CVD and the substrate that receives the copper is not electrically biased. Clearly, there is critical need for stable copper sources which are free of oxygen, fluorine, and halides, and which are compatible with aluminum precursors to prevent any cross-contamination effects during film growth.
It is especially desirable that the process and apparatus allows for the preparation of the above-mentioned films in-situ, i.e., without the necessity of transferring a substrate coated with a single film (Al or Cu) to another reaction chamber to deposit the other film. As is known in the art, a process which allows either in-situ deposition of sequential bilayers of Al and Cu followed by in-situ annealing, or in-situ simultaneous deposition of copper-doped aluminum is desirable in part because of the high affinity of aluminum for oxygen. This affinity leads typically to contamination of the Al film surface during transfer to a second reaction chamber where it is coated with Cu. The oxidized aluminum surface interferes with annealing of aluminum and copper.